In the conventional process for electroplating a layer of metal on the surface of a metal object, a consumable anode of the electroplating metal and a cathode comprising the metal object being electroplated are placed in an electrolyte solution (the electroplating bath) containing a cation of the electroplating metal, and an electrolyzing current is passed through the electrolyte between the anode and the metal object (the cathode) to cause electroplating metal to dissolve into the electrolyte at the anode and to electrolytically deposit an electrolytically equivalent amount of said metal on the surface of the cathode. The thickness of the metal layer being deposited on the surface of the cathode may vary from one point or area of the cathode to another due to variety of factors including the resistivity of the solution, the presence of gases in the solution or on the surface of the electrodes, the shape and spacing of the anode and the cathode, variations in temperature and in current density, and the like. Nonetheless, in most cases a layer of electrodeposited metal of relatively uniform thickness that is satisfactory for most purposes can be obtained throughout the entire area of the cathode being electroplated by appropriate design and placement of the anode and cathode and by appropriate control of bath temperature, current density and other electrolytic conditions of the electroplating system. However, in other cases where the layer of electrodeposited metal must be of a precise and uniform thickness in order to meet the stringent working tolerances required of the part being plated, or where the shape of the part or the physical relationship of the anode and the part present special problems, the aforementioned conventional procedures are not always sufficient to insure the production of a layer of electrodeposited metal of the required uniform thickness. In such other cases, special procedures must be devised to insure the reliable electrodeposition of metal layers of acceptably precise thickness.
For example, in the precision electroplating of the cylindrical inner surfaces of such tubular metal objects as engine cylinder liners, gun barrels, deep well pumps and the like, the consumable anode of the electroplating system is a metal wire or rod that is positioned centrally within the cylindrical wall of the tubular object, the cathode of the system being the inner surface of the tubular object being electroplated. The electrolyte solution is disposed in the annular space between the anode and the cathode, and a predetermined amount of an electrolyzing current is passed between the two electrodes to cause a predetermined amount of electroplating metal to dissolve at the anode and an equal amount of this metal to deposit on the surface of the cathode. Despite the utmost care in the placing of the anode centrally within the tubular cathode and in the control of the electrolytic conditions, the thickness of the layer of metal electrodeposited on the inner surface of the cathode will vary slightly from one point to another due to to the effect of one or more of the disruptive factors previously mentioned. Moreover, the minor differences in thickness that do develop become progressively greater as long as the electroplating operation continues and metal from the entire surface of the consumable anode continues to dissolve into electrolyte solution and to be deposited on the entire surface of the cathode being plated. That is to say, as long as metal is supplied to the electrolytic solution throughout the entire area of the consumable anode that is exposed to the solution, the same disruptive factors which initially cause the minor variations in the thickness of the layer of metal deposited on the cathode continue to operate and thereby to accenuate and increase these variations in thickness.
After an intensive investigation of the problems involved in the precision electroplating of metal objects, and in particular plating of the cylindrical inner surfaces of such metal objects as are referred to above, I have discovered that a layer of metal of predetermined uniform thickness can be reliably electrodeposited on the surface of a metal object by means of a composite anode of unique construction that, when used in an electroplating system of the type described above, automatically terminates the electrodeposition of metal on the surface of the cathode when the layer of metal reaches the desired uniform thickness.